1. Field of the Invention
The present invention relates to a process for the hydrothermal conversion of a solid starting iron compound selected from the group consisting of iron oxides, iron hydroxides, iron oxyhydroxides, and mixtures thereof into a solid product iron compound, the solid product iron compound having different physical, chemical, and/or structural properties from the solid starting iron compound.
2. Prior Art
It is known from the prior art to convert goethite (alpha-FeOOH) into hematite (alpha-Fe2O3) in a suspension under hydrothermal conditions. Romanian Patent Application RO 86979 discloses this conversion using an autoclave and temperatures of about 210° C. The same conversion, but now in the presence of potassium carbonate using temperatures between 180 and 210° C. and pressures between 16 and 20 atm., is disclosed in RO 100113.
In order to minimise operational costs and to maximize energy conservation, it would be desirable to conduct the hydrothermal conversion of iron oxides, iron hydroxides, and iron oxyhydroxides in a continuous mode. However, as will explained below, processes involving suspensions are not easy to conduct continuously, mainly due to segregation and sedimentation.
Suspensions consist of a continuous phase, i.e. a liquid, and a dispersed phase, i.e. solid particles. Suspensions can be homogeneous or heterogeneous. In this specification, homogeneous suspensions are defined as suspensions having a constant volume fraction of the continuous phase throughout the whole system. Suspensions without such a constant volume fraction of the continuous phase are referred to as heterogeneous. In these heterogeneous systems there are concentration gradients of the dispersed phase.
Suspensions can separate into a fraction with a higher volume fraction of the continuous phase and a fraction with a lower volume fraction of the continuous phase. Within this specification this phenomenon is referred to as segregation. Segregation can occur by the action of various forces, for instance centrifugal forces or gravity. Sedimentation is a form of segregation where the dispersed phase settles by gravity.
When a sediment is formed, part of the flow region within a reactor is blocked by a stagnant solid, reducing the volume available for free flow. With constant mass flux, the suspension will have to move through a smaller area, resulting in higher velocities of the continuous phase. This results in even more segregation and a non-ideal residence time distribution of the dispersed phase in the reactor.
The conversion of solid particles in a suspension can be performed continuously in traditional pipe reactors or cascade reactors, provided that the starting particles easily form a stable homogeneous suspension, e.g., a sol or a gel, and are of a more or less uniform particle size. Even then limitations in the Solids to Liquid Ratio (SLR) may occur due to the rheological behaviour of the homogeneous suspension. High energy input, e.g., high-shear mixing, can alleviate these difficulties if the suspensions exhibit shear-thinning behaviour.
Unfortunately, readily available iron oxides, iron hydroxides, and iron oxyhydroxides are not easily suspendable and/or do not form stable homogeneous suspensions, neither at high solids to liquid ratios nor at low ones. This is due either to their large particle size (say >0.1 micron) and/or to their chemical incompatibility with the liquid, making segregation of the particles from the liquid very likely. This means that the particles will show a tendency to form a sediment layer, resulting in an uncontrolled and non-ideal residence time distribution in the reactor, thereby hindering the conversion. This situation may be further aggravated when dealing with starting particles of different sizes.
Contrary to the case of the stable homogeneous suspensions described above, where high shear can assist in homogenization and reduction of the viscosity, unstable suspensions tend to segregate even faster when a high energy input is added to the system. Therefore, good mixing throughout the whole reactor and avoiding any dead or non-mixing zones is preferred to avoid non-ideal residence time distributions and to promote efficient conversion of the starting particles.
Alternatively, expensive chemicals need to be added in order to stabilize and disperse the suspension and to prevent segregation.
It has now been found that solid starting iron compounds selected from the group consisting of iron oxides, iron hydroxides, iron oxyhydroxides, and mixtures thereof can be converted continuously, even at high Solids to Liquid Ratios (SLR), in one or a series of separate vessels without an unacceptable level of segregation. Using high Solids to Liquid Ratios enables the use of relatively compact equipment and offers low operational costs and energy consumption.